The present invention relates to hybrid organic-inorganic semiconductor devices and to sensors for chemicals and light based thereon.
Sensors, i.e. devices used to locate or detect chemicals or energy, are measured by their sensitivity and selectivity. The design of sensors is aimed also at achieving robustness and versatility. Combining these qualities in the process of designing a sensor, was proven to be very difficult. The use of organic molecules in sensors has the benefit of versatility and selectivity, but is not associated with robustness, mainly because in many cases electron flow through the organic medium is required thus causing the destruction of the organic medium and therefore limiting the xe2x80x9clife timexe2x80x9d of sensors of this type.
In those cases where electron flow through the organic medium is not required, sensitivity becomes the limiting parameter. Generally, sensitivity of sensors is proportional to the contact area of the sensitive: surface, because the larger the area the higher the probability that a molecule or photon can be detected by the sensing surface. Thus, in general, sensitivity is assumed to scale with surface area.
The use of semiconductor devices as sensors is known for many years [reviewed by Mandelis and Christofides, 1993]. In general, the presently used semiconductor sensors are based on two schemes: the chemicals are adsorbed either on the gate metal and insulator layers of a field effect transistor (FET) [see, for example, U.S. Pat. No. 4,777,019, Japanese Patent publications Nos. 62163960 and 95107529] or on the surface metal of Schottky diodes.
The electronic properties of a semiconductor surface can be affected by simple chemical treatments [Skromme et al., 1987; Sandroff et al., 1987; Yablonovitch et al., 1987]. The ability to change electronic properties of a semiconductor surface by adsorption of tailor-made organic molecules to achieve selectivity [Rickert et al., 1996] has been demonstrated in the past [Lisensky et al., 1990; O""Regan and Graetzel, 1991; Bruening et al., 1994; Bruening et al., 1995; Lunt et al., 1991]. Thus, combining semiconductors with organic molecules is an attractive option for sensors, as it can combine selectivity and sensitivity. However, whenever organic molecules are involved, robustness remains an issue. In addition, there is a problem of retaining the versatility as the organic molecule needs to be attached in a reproducible and secure manner to the semiconductor.
In applications based on FET-type sensors, at least one of two basic features are found: the first feature is the third electrode (gate) that is located between the two main current-carrying contacts (source and drain) and is used to control the current through the device, in which case the sensor is based on changing the current passing through the device due to adsorption of molecules on the gate [see, for example, U.S. Pat. No. 4,777,019, U.S. Pat. No. 4,992,244, JP 62163960 and JP 95107529]. The other feature is found in the case that an ungated FET is used, a layer is adsorbed between the source and drain, in which case the sensors are characterized by a semiconductor substrate of one conductivity type, having at least two spaced apart regions of opposite conductivity type [U.S. Pat. No. 3,831,432]. These ungated devices suffer from oversensitivity to electrical interference due to their open gate structure, leading to unwanted high noise levels compared to gated devices and explaining the lack of interest in them [see Mandelis and Christofides, 1993].
Earlier proposed FET-based sensors mainly use silicon. This explains the preference for metal-oxide-semiconductor FET (MOSFET)-based structures as the relatively low barrier height that characterizes Si-devices, leads to high leakage currents unless configurations such as MOSFETs are used. An intrinsic problem one faces is the oxidation layer on the surface that reduces the sensitivity to adsorbed chemicals. By adsorbing chemicals directly onto the surface it is possible to achieve higher sensitivity. Contrary to Si, GaAs surfaces do not have a passivating native oxide layer. Hence it is possible to chemically modify the surface states, their charge, and thus the internal field at the space charge layer.
However, one of the main problems with semiconducting GaAs and related materials is that they do not have a stable native oxide film. This well known instability of their surface would seem to exclude their use as sensors because of problems with reproducibility and noise.
The surface states of GaAs are passivated when treated with sulfur (Oh et al., 1994; Besser and Helms, 1988). However, pinning of the Fermi level in GaAs has discouraged their use as chemical-sensitive devices. On the other hand, the special electronic properties of GaAs are eminently suited for the design and fabrication of structures with high electron mobility (i.e. even small changes in carrier concentration become measurable electrically) and in which charge carriers flow at a well-defined distance near the surface. The latter properties can convey onto these structures very high sensitivity to changes in surface potentials.
It has now been found, in accordance with the present invention, contrary to expectations by those skilled in the art, that it is possible to use unprotected surfaces of GaAs and related materials for the construction of sensors, even after their exposure to ambient, and get stable, sensitive sensing behavior, after special organic molecules are adsorbed on them, these organic molecules both serving as sensors and at the same time stabilizing the properties of the surfaces.
It is an object of the present invention to provide the use of organic molecules that chemisorb in the ambient directly onto specially designed GaAs structures. GaAs was used, notwithstanding its notoriously unstable surface, because a way was developed according to the invention to stabilize the surface in ambient via chemisorption of multifunctional molecules, and because structures using GaAs and (Al,Ga)As can be made extremely sensitive to surface potential changes.
The present invention thus relates to a hybrid organic-inorganic semiconductor device composed of one or more insulating or semi-insulating layers (1), one conducting semiconductor layer (2), two conducting pads (3), and a layer of multifunctional organic molecules (4), characterized in that said conducting semiconductor layer (2) is on top of one of said insulating or semi-insulating layers (1), said two conducting pads (3) are on both sides on top of an upper layer which is either said conducting semiconductor layer (2) or another of said insulating or semi-insulating layers (1), making electrical contact with said conducting semiconductor layer (2), and said layer of multifunctional organic molecules (4) is directly bound through at least one of said functional groups to the surface of said upper layer, between the two conducting pads (3), and at least another of said functional groups of said multifunctional organic molecules binds chemicals or absorbs light.
The multifunctional organic molecule used in the structure of the invention has at least one functional group that binds to the surface of the upper layer, said group being preferably selected from one or more aliphatic or aromatic carboxyl, thiol, sulfide (e.g., methylsulfide, acyclic disulfide, cyclic disulfide), hydroxamic acid and trichlorosilane (for binding to silicon oxide) groups, and at least one second functional group that binds chemicals or absorbs light. Examples of functional groups that bind metal ions and are suitable for detection of metal ions such as Cu2+, Fe2+, and Ru2+, are, without being limited to, radicals derived from hydroxamic acid, bipyridyl, imidazol and hydroxyquinoline. Examples of functional groups that are efficient light absorbers at given wavelenghts and are suitable for detection of light include, without being limited to, aliphatic and aromatic hydroxamates, substituted aromatic groups such as cyanobenzoyl and methoxybenzoyl bipyridyl groups (that can bind Ru2+), hydroxyquinoline groups, imidazolyl groups to which a metal porphyrin or a metal phthalocyanin residue is attached, etc.
Examples of multifunctional organic molecules that can be used as light sensors according to the invention are 2,3-di(p-cyanobenzoyl) tartaric acid (DCDC), 4,5-di(p-cyanobenzoyloxy)- 1,2-dithiane (DCDS), 4,5-di(p-methoxy-benzoyloxy)-1,2-dithiane (DMDS) and 1,2-dithiane-4,5-di(hydroxyquinoline) and the Cu+ complex thereof.
The multifunctional organic molecules may further contain functional groups that will control the fundamental properties of the semiconductor surface, such as electron donating or electron withdrawing groups such that a dipole layer with a defined direction is formed on the surface, and/or functional groups that control the sensitivity to certain species, change the hydrophobicity or hydrophilicity of the surface, etc.
In one embodiment, the conducting semiconductor layer (2) of the structure of the invention is a semiconductor selected from a III-V and a II-VI material, or mixtures thereof, wherein III, V, II and VI denote the Periodic Table elements III=Ga, In; V=As, P; II=Cd, Zn; VI=S, Se, Te. This conducting semiconductor material is preferably n-GaAs or n-(Al,Ga)As doped, for example, preferably with Si. In another embodiment, the one or more insulating or semi-insulating layers (1) of a device of the invention, that may serve as the base for the device, is a dielectric material selected from silicon oxide, silicon nitride or from an undoped semiconductor selected from a III-V and a II-VI material, or mixtures thereof, wherein III, V, II and VI denote the Periodic Table elements III=Ga, In; V=As, P; II=Cd, Zn; VI=S, Se, Te, and is preferably an undoped GaAs or (Al, Ga)As substrate.
The invention further provides sensors for chemicals and light based on the devices of the invention.